The present invention relates to the field of electronic technology, particularly to a low current single-phase ground fault detection and location methods and systems.
Power system contains power supply from power plants. Power is transferred to the load side first by high or extra high voltage transmission network, and then by the lower voltage levels of distribution network.
A common fault in the main power grid is short circuit and ground fault. Short circuit faults include three-phase and two-phase short circuit faults. A common ground fault type is a single-phase ground. Short circuit fault detection technology has been very mature. But for single-phase ground fault detection, especially for low current grounded distribution network single-phase ground, there is no effective method, and is recognized as a worldwide problem.
China and some countries are using mostly low current grounded grid distribution network. Therefore, the vast majority of failures are single-phase ground. The main advantage of low current grounded distribution grid is that: in a single-phase ground fault, unlike a short circuit, the system produces only a low ground current, therefore, the three-phase line voltage is still symmetrical, and does not affect the normal operation of the system. China regulations require if a single-phase ground fault occurs, the low current grounded distribution network should continue to operate with fault 1˜2 h. Because of this reliability, the low current grounded distribution network has been widely used.
However, single-phase ground fault must be found as quickly as possible during single-phase ground fault troubleshooting. Otherwise, the over voltage caused by the failure of the ground fault can cause the cable to explode, the voltage transformer PT to burn down, the bus to burn and other power system accidents. If not fixed in time, long-running ground fault will burden local residents, livestock with tremendous security risks.
The grounded mode of the low current grounded power distribution network is either not grounded or grounded by the arc suppression coil. According to DL/T 1997-620 “AC electrical equipment over voltage protection and insulation coordination”, in the 10 kV distribution network, which is composed of pure overhead line or overhead line and cable, if the capacitor current is less than 10 A, it can be used ungrounded. But when the capacitor current is greater than 10 A, it is necessary to install the arc suppression coil.
Distribution network with either neutral point ungrounded or arc suppression coil grounded mode, when the single-phase ground occurs in, does not produce a large fault current in A, B, C phases, the following details the two cases:
1 Distribution Network with Neutral Point Ungrounded Method
When the distribution network with neutral point N is not grounded, as shown in FIG. 1, in one of the three phases A, B, C, such as A phase, assume a ground fault occurs. In the initial phase failure, phase A voltage will drop rapidly, voltages of non-fault phase B and phase C will rise rapidly, the neutral point voltage also will rise rapidly. While the A-phase feeder ground distributed capacitance will take place quickly discharging to ground, B phase and C phase feeder ground distributed capacitance rapidly charged through the ground to form a short-lived charge-discharge process (10 ms˜20 ms), have a relatively large transient capacitive current. Than the system will enter into steady state power by way of the non-fault phase distributed capacitance and ground, resulting in a continuous steady state capacitive current. In this process, the transient capacitive current is much larger than the steady-state capacitive current.
Adding all the three-phase line currents, one can get the line zero-sequence current. Similarly, adding the three-phase line voltages together, one can get the line zero-sequence voltage. In this process, the fault line zero sequence current is shown in FIG. 2. Before a ground fault occurs, the line zero-sequence current is very low, approximately zero. When a ground fault occurs, first there is the capacitor charge and discharge transient process, resulting in a relatively large high frequency transient capacitive current, and then to maintain a major energy concentrated in the frequency (50 Hz or 60 Hz) of the steady state capacitive current.
2 Distribution Network Neutral Point Petersen Coil Grounded Method
When the distribution network with neutral point N grounded through the Petersen coil, shown in FIG. 3, in one of the A, B, C three phases, for example, phase A, assume a ground fault occurs. Similar to the ungrounded method, in the initial part of the failure, phase A voltage will drop rapidly, voltages of non-fault phase B and phase C will rise rapidly. Also will rise is the neutral voltage. Next, the A-phase feeder ground distributed capacitance quickly discharged to ground through grounded points. B phase and C phase feeder ground distributed capacitance rapidly charged through the ground to form a short-lived charge-discharge process (10 ms˜20 ms), forming a relatively large transient capacitive current. Next, the suppression coil L will produce a compensating current to compensate for the non-fault phase power through the distributed capacitance, and in the process, forming ground capacitive current comparable to a steady state. Last, the system enters the steady state. Using neutral point arc suppression coil grounded method, the steady capacitance fault line current will become very low, and unlike FIG. 2, no large steady-state zero-sequence current is produced. But in the fault line, the transient capacitive current is not affected.
In low current grounded method, in particular by Petersen coil, in the distribution network single-phase ground fault situation, the instantaneous fault current duration is very short, and the steady-state fault current is very low. Data shows that the vast majority of single-phase ground fault grounded resistance is greater than 800Ω, belongs to the high-impedance ground fault, such as through the branches, the grass, the damp earth walls, etc., such that the instantaneous fault current is not large, with very short duration of 10˜20 ms. Thus in a low current grounded method with a single-phase ground fault, the detection and location is recognized as a worldwide problem. There are several methods and apparatus for low current single-phase ground fault detection:
1 Substation Low Current Grounded Line Selection Means
Existing low current grounded line selection device allows the substation bus to identify which line ground fault has occurred. In FIGS. 1 and 3, for example, there are two lines—a fault line and a non-fault line. Through a low current grounded line selection device, one can identify the fault line.
A low current grounded line selection device works by monitoring zero sequence current residual voltage at substation bus and each branch line. By treating a sudden increase in the zero-sequence voltage in the grounded line as a trigger condition, follows by using the various branches of the line the zero-sequence current steady state information and transient information, the device will identify the fault line. According to the use of different information, the device operation can be classified as of steady line mode or transient line mode.
Steady line determination is mainly based on:
(1) The zero sequence current amplitude is maximum at the fault line;
(2) The fault line's zero-sequence current phase is opposite of that of the non-fault line;
(3) The fault line's zero-sequence reactive power is negative;
(4) The fault line's zero-sequence active power is large;
(5) The current of 5th harmonic is large at the fault line and is opposite of that of the non-fault line;
(6) The fault line's negative sequence current is large.
Transient line determination is mainly based on:
(1) The non-fault line and the fault line differs in that when phase voltage reaches zero-sequence, the transient current and voltage of the first half-wave amplitude and phase are different;
(2) The use of other processing methods such as from the zero-sequence current characteristics of transient information, extracting a low wave, and from there, with artificial intelligence, such as neural networks, identifying the fault line and non-fault line.
The main disadvantages of the low current grounded line selection device are:                (1) The existing substation PT (voltage transformer) and CT (current transformer) affect the reliability and accuracy of the selected line.        
For the low current grounded line selection device to trigger, the bus zero-sequence voltage signal must be connected in parallel on the bus PT to be obtained. Selection device is subject to PT ferromagnetic resonance, and it will cause significant interference.
Because factors such as the special zero-sequence CT volume, high cost, and the need to install power, for low current grounded line selection device to obtain zero sequence current is not usually obtained through special zero-sequence CT, but by a substation having three-phase or two-phase measurements obtained with CT. The ideal CT, is one with no magnetizing current consumption, the ampere-turns of the primary and the secondary coils are equal in value, the primary current and the secondary current phase measurements are the same and there is no phase shift. In a practical CT, there is an excitation current, thus the ampere-turns of the primary and the secondary coils are not equal, and the primary current phase and the secondary current phase are not the same. Therefore, due to the actual CT usually having angular error and phase change error, resulting in an unbalanced three-phase CT. Also the three-phase CT superposition of zero-sequence current is an unbalanced current, and the actual zero-sequence current errors exist, that would impact of the line election results. In addition, in traditional measurement of CT, due to the magnetic core having non-linear excitation characteristics, there is an impact on the current linearity from low current to large current. When the current is large, the core exhibits magnetic saturation, which will lead to CT saturation. In practice, the mid to low voltage grid frequently exhibits the CT saturation phenomenon, causing failure in getting the correct zero sequence current in performing line selection. Moreover, the core maintains energy storage cycle and magnetizing cycle, which makes CT transient characteristics not satisfactory. When current changes are not properly followed, it is difficult to accurately capture weak transient signals.                (2) The location of the ground fault cannot be accurately located.        
Low current grounded line can only be installed in power distribution bus to sub location, and only for selecting a ground fault in the branch line. It is not capable to locate the position of branch line where the ground fault occurs.
2 Signal Injection Method and Apparatus
Signal injection method through the injection signal source with fault detection and location indicator may be used for detecting a large ground current on a permanent ground fault. The principle of this method is: when the substation detects that the zero-sequence voltage increases significantly and continue for some time, together with the zero-sequence current being greater than a threshold value, and continue for some time, it can be determined that the ground fault occurs; upon this determination, Petersen coil is required and inserted; thereafter, by inserting at the neutral point of the transformer with a certain pattern of fault current signal; because the fault current signal can be detected by the fault indicator at a location before the ground fault, but not at a location after the ground fault, the location of the ground fault can be traced.
As seen in FIG. 4, the ground source is connected between the neutral of the grounded substation transformer and ground with controlled resistive loads (mid-range resistance, typically 100Ω). When ground fault is detected, using micro controller, at the neutral of the grounded substation transformer (in the case of non-grounded transformer, the bus neutral point), the resistive load source is automatically deployed for a short period of time, so that between the substation and field ground a special coded low signal current is produced. By resistive load switching with ground source code control, it can generate a superimposed load current embedded with coding in the current signal. By installing the ground fault indicators though out at the line and at the branch point of the substation, the detection of the current signal could be automated, and therefore, the goal of locating the fault is achieved.
Injection signal source method has the following disadvantages:
(1) It requires the installation of the signal source in the substation, therefore, changing the system operation;
(2) The signal sources and other devices require additional investment and construction, and the construction process requires a power outage;
(3) For ground fault at the common resistance of 800Ω or more, this method was unable to produce a sufficient large coded signal current to be detectable by fault indicator, therefore, this method cannot detect high-resistance ground fault;
(4) It cannot detect transient ground faults.
3 Network Feeder Terminal Unit FTU Based Method for Ground Fault Detection and Location
FTU network based ground fault detection and location method is shown in FIG. 5. The method works by installing switches and related detection terminal FTU throughout the line and wiring them in a network, so to record three-phase current, voltage waveform data during ground fault and send the data to the automated central system to be analyzed in order to determine within which switch the fault lies.
The main disadvantages of the FTU network based ground fault detection and location method is:
(1) There is a need to install the switches, and the switches must have an internal CT and PT, with switches and FTU being a huge investment;
(2) In measuring CT, it would be difficult to capture the transient signals; also it would be facing high current saturation; and during the three-phase superimposed zero-sequence, phase imbalance is causing large errors; these are factors making high resistance grounded difficult to be detected;
(3) PT has ferromagnetic resonance problem;
(4) Installing switch and FTU needs power lines to be offline;
(5) It can only locate the fault up to between switches, but not more in terms of accurate positioning;
(6) For overhead line to provide power to the FTU is very difficult, negatively affecting its installation and functioning.
At present time, the low current grounded system, especially the neutral point Petersen coil grounded system, the lack of effective ground fault detection methods and equipment makes it hard to detect the ground fault position. Many electricity departments are still using manual diagnostic methods such as cable method to locate the ground faults. These methods are of low degree of automation, are difficult and inefficient to implement, and are unable to meet the requirements of the power system to continue to improve the reliability of power supply. These methods are also making it difficult to improve power quality and power supply reliability. In order to improve the distribution network for low current grounded power supply reliability, it is necessary to provide a method and apparatus, when the single-phase distribution network has aground fault, whether it is based on low resistance or high resistance grounded, or whether it is instantaneous fault or a permanent fault, to effectively detect and locate the fault.